Wells are generally drilled into the ground to recover natural deposits of hydrocarbons and other desirable materials trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a drill string. The well is drilled so that it penetrates the subsurface formations containing the trapped materials and the materials can be recovered.
At the bottom end of the drill string is a “bottom hole assembly” (“BHA”). The BHA includes the drill bit along with sensors, control mechanisms, and any required circuitry. A typical BHA includes sensors that measure various properties of the formation and of the fluids that are contained in the formation. A BHA may also include sensors that measure the BHA's orientation and position.
Drilling operations are controlled by an operator at the surface. The drill string is rotated at a desired rate by a rotary table, or top drive, at the surface, and the operator controls the weight-on-bit and other operating parameters of the drilling process.
Another aspect of drilling and well control relates to the drilling fluid, called “mud.” The mud is a fluid that is pumped from the surface to the drill bit by way of the drill string. The mud serves to cool and lubricate the drill bit, and it carries the drill cuttings back to the surface. The density of the mud is carefully controlled to maintain the hydrostatic pressure in the borehole at desired levels.
Some drilling systems use a “mud motor” to rotate the drill bit. A mud motor is a device in the BHA that converts some of the fluid power in the downward flow of mud into rotational motion. With a mud motor, the drill bit may be rotated without having to rotate the entire drill string from the surface. Commonly used mud motors include turbine motors and positive displacement motors.
In order for the operator to be able to control the direction of the drill bit or to control downhole sensors or instruments, communication between the operator at the surface and the BHA are necessary. A “downlink” is a communication from the surface to the BHA. Likewise, an “uplink” is a communication from the BHA to the surface. Based on the data collected by the sensors in the BHA, an operator may desire to send a signal via downlink to the BHA. A common downlink signal is an instruction for the BHA to change the direction of drilling or to perform a test or collect data. Downlink signals are also used to activate and deactivate various MWD sensors while the borehole is being drilled. During borehole workover operations, various types of downhole or bottom equipment are activated by downlink signals from the surface.
There are various prior art downlink methods. One class of downlink methods is called “mud pulse telemetry.” Mud pulse telemetry uses pulses in the mud flow rate or pressure to communicate with the BHA.
One method of mud pulse telemetry uses the mud pumps at the surface to control the mud flow rate to the BHA. The flow rate is detected and interpreted by the downlink system. This may be accomplished using a mud turbine generator in the BHA. The amount of power generated by the turbine is related to the mud flow rate. Alternatively, the mud flow rate can be determined by monitoring the rates of rotation of a positive displacement mud motor (“PDM”) or a drilling mud turbine (called a “turbodrill”). For example, U.S. Pat. No. 4,647,853 issued to Cobern discloses a system for detecting the rate of rotation of a downhole turbine using a triaxial magnetometer, which is commonly used to define the location and orientation of the downhole drilling assembly. A powerful permanent magnet is mounted on the uphole end of the turbine drive shaft, with the magnetic moment of the magnet perpendicular to the axis of the turbine shaft. As the turbine shaft rotates, this turbine mounted magnet superimposes a rotating magnet field on the earth's magnetic field in the vicinity of the turbine. This superimposed rotating field constitutes a mud motor tachometer signal, which is sensed and separated from the response of the system's existing magnetometer. The signal defines the rotation rate of the mud motor turbine, and hence the mud flow rate.
Another method of mud pulse telemetry uses pressure pulses for communicating with the BHA. A pressure pulse is transmitted from the surface, and pressure sensors in the BHA detect and interpret the pressure pulses generated at the surface.
Other methods for downlink communication include changing drill string rotation rates. For example, U.S. Pat. No. 4,763,258 issued to Engelder discloses methods and apparatus for telemetering while drilling by changing drill string rotation angle or drill string rotation rate or rotation “speed”. The magnitude of an incremental rotation of the drill string is related to a downhole function, such as the activation of a specific downhole sensor. The incremental rotation is sensed by a downhole inclinometer and magnetometer, which are normally carried by a deviated hole downhole drilling system to define the orientation and location of the downhole equipment.
Similarly, U.S. Pat. No. 6,267,185 issued to Mougel, et al., discloses downlink methods by rotating drill string by discrete angles. The sequence of discrete angular rotations is sensed downhole by a gyroscope and decoded as a command in a microprocessor. An alternative method involves rotating the drill string by different angular rates, which are likewise sensed by the gyroscope and decoded in the microprocessor. The microprocessor then transmits the decoded command to the controlled equipment.
While these prior art methods are capable of providing downlink communications, there is still a need for more reliable downlink systems that are capable of providing improved quality and speed of downlink communications.